Stripe.com payment integration module

[CaroleAs]

I have a customer who used to use a card number (call it "old"). That card has been replaced by a "new" one, that works fine outside of my store.
On the checkout, even though she enters the "new" card number manually, it still tries to use the "old" card.

She unchecks the "use the default card...", but it re-checks it when she tries to check out
She cleared the cache on her computer
On her Stripe account, I even removed the default card so there is nothing, and the "old" card does not appear anywhere.
Yet, it still tries to use it when she checks out.

So my question is whether the "old" card information is somewhat stuck in the checkout. Is there a way to "clear the cache" on that?
(using ZC 1.5.7c)